Electronic data processing systems employ complex manipulations of data and instruction words stored in memory which involve the transfer of logic signals from one circuit subsystem to another. The effect of spurious noise, for example, supply voltage variations and input variations induced by switching transients, has become an increasingly important design consideration, particularly in circuits in which analog and digital subsystems are interfaced in an integrated circuit, and where transistor-transistor logic (TTL) circuits are coupled in driving relation to metal-oxide-semiconductor (MOS) circuits.
TTL logic circuits generate current transients or spikes because of their stacked output structure. When the output of a TTL logic circuit is switching from the low state to the high state, there is a short interval of time during the switching transition where the switching transistors are conducting and a relatively large surge of current (30-50 mA) is drawn from the +5 volt supply (V.sub.cc). The duration of this current transient is extended by the effects of any load capacitance on the circuit output. The load capacitance includes stray wiring capacitance and the input capacitance of any load circuits and will be charged up to the high state output voltage. Consequently, when a TTL output circuit goes from low to high, a high-amplitude current spike will be drawn from the V.sub.cc supply.
In a complex digital circuit, there may be many TTL outputs switching states at the same time, with each one drawing a narrow spike of current from the power supply. The cumulative effect of all these current spikes will be to produce a voltage spike on the common V.sub.cc line, mostly due to the distributed inductance on the supply line. Moreover, stray electrical and magnetic fields can induce spurious voltages on the connecting wires between logic circuits. These unwanted, spurious signals can sometimes cause the voltage at the input to a logic circuit to drop below V.sub.IH (MIN) or rise above V.sub.IL (MAX), which can produce unpredictable operation.
For example, spurious noise on supply voltage or coupling of signals between adjacent input pins can result in falsely interpreted input signals. V.sub.IH may become V.sub.IL for a moment, which can falsely trigger counters, shift registers, pointers and the like. This is a non-recoverable event and can cause catastrophic consequences such as inhibiting or masking of valid data or instruction words or the transmission of an erroneous data or instruction word.
The input voltage requirements of a logic circuit are illustrated in FIG. 2. The logic circuit will respond to any input voltage greater than V.sub.IH (MIN) as a logic 1, and will respond to voltages lower than V.sub.IL (MAX) as a logic zero. Input voltages in the indeterminate range will produce an unpredictable response and should not be allowed. When a logic high output is driving a logic circuit input, a negative noise spike appearing on the signal line may cause the input voltage to drop into the indeterminate range. When a logic low output of one circuit is driving a logic input of another circuit, a positive noise spike on the signal line can drive the input voltage into the indeterminant range, where an unpredictable operation can occur.